mrs_over, it is the fact that polygamy is allowed in some countries and not in others, look at the situation as I'm explaining it.
Don't be too personnel with this.
Jog is spot on with this
Putting aside other cultures people can know about other partners and carry on OK in full knowledge of them
President Mitterand had an illegitimate daughter who the public only found out about at his funeral although everybody close to them all knew about each other for years.
Knowing about each other doesn't make it OK people can be manoevered into situations they end up accepting even though they're not happy about it.
Usually one partner has too much invested just to walk away - the Queen and the Royal mistress scenario.
Cabinet ministers for a more down to earth example. If her unfaithful husband is exposed he loses his job and she loses out financially and the status she enjoys through his position.
In some cases with older people one partner loses the drive whilst another's is strong and the first overlooks the other's infidelities whilst they don't involve emotional attachments and the neighbourhood doesn't find out.
For many people it's more complicated than slamming the door and walking out.
The commitment you'll be promising to those wives/ or g/fs being that YOU'LL NEVER EVER be found cheating with a third person, after having the two that knows and understands one another.